Automatically switched redundant switch configurations

ABSTRACT

A branch unit for a fiber optic system that includes a service path and a protection path, whereby the branch unit provides switching to account for problems due to fiber cuts and/or equipment failures that may occur in the fiber optic system. The service and protection paths meet at a branch point of the fiber optic network, or at a network protection equipment (NPE) that is located near a customer interface equipment. A plurality of switches are provided at the branch unit or NPE, along with a detector and a processor, to determine whether any signals are being received from the service path, and if not, to reconfigure the system to accept signals from the protection path.

BACKGROUND OF THE INVENTION

[0001] A. Field of the Invention

[0002] The invention relates generally to redundant switchconfigurations, and more specifically to redundant switch configurationsthat provide both signal loss protection and equipment failureprotection.

[0003] B. Description of the Related Art

[0004] For fiber optic networks, problems in transmitting and receivingsignals may be due to equipment failure, such as switch failure, or itmay be due to failure of the signal lines, such as the fiber optic lineswhich provide signals from a source to a destination.

[0005] Typically, conventional optical communication systems comprise areceiving node and a transmitting node (Baltimore, Md. and New York,N.Y., for example) connected via optical fiber. Each node containsequipment for communication via optical fiber. Such equipment mayinclude channel equipment and Wavelength Division Multiplex (WDM)equipment. Channel equipment is equipment that transmits and receivesvia a specific wavelength (or channel). In a conventional system, if afiber is cut resulting in a loss of signal, the system requires anetwork element (such as a SONET processor) to determine there is afailure in the digital domain and notify the switch to change state.

[0006] Further, switches are utilized to direct signals transmitted bythe nodes to various fiber optical cables within a conventional opticalcommunication system. When a switch fails in a conventional system, anoperator manually reconfigures the switch to communicate via analternate channel. The resulting down time from manually switchingchannels results in a high amount of data loss and an inefficient use ofbackup resources.

SUMMARY OF THE INVENTION

[0007] The present invention is directed to overcoming or at leastreducing the effects of one or more of the problems set forth above, aswell as other problems found in the prior art.

[0008] In a first aspect of the present invention, a fiber optic systemis provided comprising a primary transmission path provided from asource, a secondary transmission path provided from the source, and anetwork protection unit coupled to the primary and secondarytransmission paths provided from the source. The network protection unitcomprises a first 1×2 switch, a second 1×2 switch, and a third 1×2switch.

[0009] The first 1×2 switch comprises a first input optically coupled tothe primary transmission path, a second input optically coupled to thesecondary transmission path, and an output. The second 1×2 switchcomprises a second input optically coupled to the primary transmissionpath, a first input optically coupled to the secondary transmissionpath, and an output. The third 1×2 switch comprises a first inputoptically coupled to the output of the first switch, a second inputoptically coupled to the output of the second switch, and an outputoptically coupled to an output transmission path.

[0010] In a first mode of operation, the first and third switches areset to provide the primary signal to the output transmission path. In asecond mode of operation, the first and third switches are set toprovide the secondary signal to the output transmission path. In a thirdmode of operation, the second and third switches are set to provide theprimary signal to the output transmission path. In a fourth mode ofoperation, the second and third switches are set to provide thesecondary signal to the output transmission path.

[0011] In another aspect of the present invention, a fiber optic systemis provided comprising a primary transmission path provided from asource, a backup transmission path provided from the source, and abranch unit provided at a meeting point of the primary and backuptransmission paths.

[0012] The branch unit comprises a first 2×2 switch, a second 2×2switch, a third 2×2 switch, and a processor. The first 2×2 switchcomprises a first input optically coupled to the primary transmissionpath, a second input optically coupled to the secondary transmissionpath, a first output, and a second output optically connected to adetector. The second 2×2 switch comprises a second input opticallycoupled to the primary transmission path, a first input opticallycoupled to the secondary transmission path, a first output, and a secondoutput optically coupled to a detector. The third 2×2 switch comprises afirst input optically coupled to the first output of the first 2×2switch, a second input optically coupled to the first output of thesecond 2×2 switch; a first output, and a second output.

[0013] The processor receives information from the detectors regardingthe detected signal strength at the second output port of the first 2×2switch and the second output port of the second 2×2 switch. The first2×2 switch operates in either a first mode that provides input receivedon its first input to its first output and input received on its secondinput to its second output, or a second mode that provides inputreceived on its first input to its second output and input received onits second input to its first output. The second 2×2 switch operates ineither a first mode that provides input received on its first input toits first output and input received on its second input to its secondoutput, or a second mode that provides input received on its first inputto its second output and input received on its second input to its firstoutput. The processor commands the first and second 2×2 switches tooperate in one of the first mode of operation and the second mode ofoperation, based on the information received from the detectors.

[0014] In another aspect of the present invention, a fiber optic systemis provided comprising a primary transmission path provided from asource, a backup transmission path provided from the source, and abranch unit provided at a meeting point of the primary and backuptransmission paths.

[0015] The branch unit comprises a first 2×2 switch, a second 2×2switch, a detector, and a processor. The first 2×2 switch comprises afirst input optically coupled to the primary transmission path, a secondinput optically coupled to the secondary transmission path, a firstoutput, and a second output. The second 2×2 switch comprises a firstinput optically coupled to the first input of the first 2×2 switch, asecond input optically coupled to the second input of the second 2×2switch, and an output optically coupled to a main transmission path. Thedetector is optically coupled to an output of the second 2×2 switch. Theprocessor is in communication with the detector for controlling thefirst 2×2 switch and the second 2×2 switch.

[0016] The first 2×2 switch operates in either a first mode thatprovides input received on its first input to its first output and inputreceived on its second input to its second output, or a second mode thatprovides input received on its first input to its second output andinput received on its second input to its first output. The second 2×2switch operates in either a first mode that provides input received onits first input to an output, or a second mode that provides inputreceived on its second input to an output. The processor commands thefirst 2×2 switch and second 2×2 switch to operate in one of the firstmode of operation and the second mode of operation, based on theinformation received from the detector.

[0017] In another aspect of the present invention, a method of providingfiber optic signals on a fiber optical network is provided, the methodcomprising the steps of providing, from a source, a primary signal on aprimary transmission path, providing, from the source, a backup signalon a backup transmission path, receiving the primary and backup signalson the primary and backup transmission paths, respectively, andoutputting only one of the primary and backup signals onto an outputport that correspond to a main optical path, by way of at least twoswitches, detecting a signal strength on the main optical path, anddetermining, based on signal strength or quality, whether or not tooperate in a first mode of operation, in which the primary signal isprovided to the main optical path, or in second mode of operation, inwhich the backup signal is provided to the main optical path.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The foregoing advantages and features of the invention willbecome apparent upon reference to the following detailed description andthe accompanying drawings, of which:

[0019]FIG. 1 is a block diagram of a network connection according to thepresent invention;

[0020]FIG. 2 is a block diagram of a branch unit according to a firstembodiment of the invention;

[0021]FIG. 3 is a block diagram of a branch unit according to a secondembodiment of the invention;

[0022]FIG. 4 is a block diagram of a branch unit according to a thirdembodiment of the invention;

[0023]FIG. 5 is a block diagram of a fourth embodiment of a fiber opticsystem with a WDM network protection equipment (NPE) array;

[0024]FIG. 6 is a block diagram of a fifth embodiment of a fiber opticsystem with a NPE array;

[0025]FIG. 7 is a block diagram of a sixth embodiment of a NPE array ofswitches;

[0026]FIG. 8 is a block diagram of a seventh embodiment of a NPE arrayof switches.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0027] An example of a network connection according to the presentinvention is shown by the block diagram of FIG. 1. Nodes 10 and 20 maybe transmitting and receiving nodes separated by a body of water. Forexample, node 10 may be a node located in Paris, France and node 20 maybe a node located in New York, N.Y. Node 10 is optically connected toprotection equipment 30 via optical fiber 90. Similarly, node 20 isoptically connected to protection equipment 40 via optical fiber 100.

[0028] In a first transmit operation mode, node 10 transmits data tonode 20 via service transmit optical fiber 60. In a second transmitoperation mode, when a fiber cut in service transmit optical fiber 60occurs, node 10 transmits data to node 20 via protect transmit opticalfiber 70.

[0029] In a first receive operation mode, node 10 receives data fromnode 20 via service receive optical fiber 50. In a second receiveoperation mode, when a fiber cut in service receive optical fiber 50occurs, node 10 receives data from node 20 via protect receive opticalfiber 80.

[0030] Protection equipment 30 and 40 provide for switching (typicallywave division multiplexed switching) between diversely routed serviceand protection optical fibers 50, 60, 70, and 80. Protection equipment30 and 40 typically comprise branch units in relatively close physicalproximity to nodes 10 and 20, and may further comprise opticalrepeaters, amplifiers, and other optical transmission related devices.

[0031] A first embodiment of a fiber optic system is shown by the blockdiagram of FIG. 2. Branch unit 285, in this block diagram depicted as areceiving branch unit, according to the first embodiment is opticallycoupled to service receive optical fiber 260 and protect receive opticalfiber 270. Service optical fiber 250 is the primary transmission pathoptically connected to a receiving node. In reference to FIG. 1, servicereceive optical fiber 260 correlates to service receive optical fiber50, and protect receive optical fiber 270 correlates to protect receiveoptical fiber 80. Service optical fiber 250 correlates to a receive pathof optical fiber 90.

[0032] Service receive optical fiber 260 obtained from a first branchpath is optically split via 50/50 optical coupler 295. Split 50/50service receive optical fiber is optically connected to a first input ofa first 1×2 switch 210 and optically connected to a second input of thesecond 1×2 switch 220. Similarly, protection receive optical fiber 270obtained from a second branch path is optically split via 50/50 opticalcoupler 290. Split 50/50 protect receive optical fiber is opticallyconnected to a second input of the first 1×2 switch 210 and opticallyconnected to a first input of the second 1×2 switch 220.

[0033] The output of the first 1×2 switch 210 is provided to a firstinput of a third 1×2 switch 230, and the output of the second 1×2 switch220 is provided to a second input of the third 1×2 switch 230. Theoutput of the third 1×2 switch 230 is coupled to the primarytransmission path 250.

[0034] A light tap 280 is provided at the output of the third 1×2 switch230, and a photodetector 240 is coupled to the light tap 280 to detectan output signal level. Information from the photodetector 240 isprovided to a processor or controller 200. Based on the informationprovided, the processor 200 controls the first, second and third 1×2switches 210, 220, 230 to be set to a particular state, either firstinput port to output port or second input port to output port.

[0035] As shown in FIG. 2, in normal operation mode, the first 1×2switch 210 is set to provide the service input on the first input portto its output port, and the second 1×2 switch 220 is set to provide theprotection input on the first input port to its output port. The third1×2 switch 230 is normally set to provide the service input on its firstinput port as provided to it by the output port of the first 1×2 switch210, to its output port. As a result, under normal operation mode, theservice path is provided to the primary transmission path opticallyconnected to a receiving node at the output of the third 1×2 switch 230.

[0036] When a failure in the service path is determined by the processor200 due to no (or less than some predetermined threshold) signalstrength being detected by the photodetector 240, the third 1×2 switch230 is switched, under control of the processor 200, to couple thesecond input port containing signals on the protection path to theoutput port of the third 1×2 switch 230. This switch effectivelymaintains the network even when a fiber cut exists on the service path.

[0037] However, if the third 1×2 switch 230 is malfunctioning in that itwill not allow itself to be set to the second input port-to-output portmode, then the first 1×2 switch 210 may be switched under control of theprocessor 200 to couple the protection signals received on its secondinput port to its output port. In this scenario, the protection signalsare received on the first input port of the 1×2 switch 200 and thenoutput onto the main optical path coupled to the output of the third 1×2switch 230.

[0038] The system according to the first embodiment can also operatewith a malfunction of the first 1×2 switch 210 by switching the second1×2 switch 220 to provide the proper signal path to the third 1×2 switch230. Thus, the branch unit 285 according to the second embodiment of theinvention is capable of maintaining network integrity even if one of the1×2 switches 210, 220, 230 fails.

[0039] In the first embodiment, a high voltage switch (not shown) isoptionally provided at the branch unit 285 so that failed legs can beshorted to ground to allow those failed legs to be repaired, asexplained in some detail above. The high voltage switch is preferablycommanded by way of the network management system, so that the leg underrepair is switched to a load (not shown) coupled to the high voltageswitch when the leg is being repaired.

[0040] Additionally, a second photodiode, light tap and processor may beprovided at the branch units according to any of the embodimentsdescribed herein, in order to provide an additional level of redundancy.For each of the embodiments described herein, failure of a service pathcan be detected very quickly since there are few if any propagationdelays, and thus the processor can be notified of (or detect) a problemon a service path and rapidly command a switch to a protection path.Reconfiguration times substantially under a few milliseconds can beachieved from first detection of a failure on a service path, toswitching to an appropriate protection path in the first and secondembodiments.

[0041] As an alternative configuration of the first embodiment shown inFIG. 2, a first photodiode may be provided at the output of the first1×2 switch 210, and a second photodiode may be provided at the output ofthe second 1×2 switch 220. The first photodiode monitors switchover tothe backup line, and the second photodiode monitors loss of signal inthe service line. If the output of the second photodiode goes below apredetermined level (thereby indicating loss of signal in the serviceline), the first 1×2 switch 210 is switched to provide the protectionsignal on its output port. The first photodiode monitors the switchoverto backup. If, after the first 1×2 switch 210 has been switched, thepredetermined level is not met, as determined by the second photodiode,the second and third switches 220, 230 are triggered, to provide theprotection signal to the main output path. Additionally, the switchoverto the backup or protection signal can be done in the first 1×2 switch210.

[0042] A second embodiment of a fiber optic system is shown by the blockdiagram of FIG. 3. This second embodiment comprises a redundant 2×2latching switch architecture for an automatically switched redundantswitch structure, utilized in a branch unit 300.

[0043] Similar to the first embodiment, service receive optical fiber260 obtained from a first branch path is optically split via 50/50optical coupler 295. Split 50/50 service receive optical fiber isoptically connected to a first input of a first 2×2 switch 380 andoptically connected to a second input of a second 2×2 switch 370.Similarly, protection receive optical fiber 270 obtained from a secondbranch path is optically split via 50/50 optical coupler 290. Split50/50 protect receive optical fiber is optically connected to a secondinput of the first 2×2 switch 380 and optically connected to a firstinput of the second 2×2 switch 370.

[0044] A first output of the first 2×2 switch 380 is provided to a firstinput of a third 2×2 switch 340, and a second output of the first 2×2switch 380 is provided to a photodetector 320. A first output of thesecond 2×2 switch 370 is provided to a second input of a third 2×2switch 340, and a second output of the second 2×2 switch is provided toa photodetector 360.

[0045] A first output of the third 2×2 switch 340 is optically coupledto the primary service transmission path 390. A second output of thethird 2×2 switch 340 is optically coupled to the secondary protectiontransmission path 395.

[0046] Under normal operating conditions, a first 2×2 switch 380receives the service signal on its first input port, and provides thatsignal to its first output port. A second input port of the first 2×2switch 380 receives the protection signal, and provides the protectionsignal to a second output port of the first 2×2 switch 380. A firstphotodetector 320, for example a photodiode, is provided at the secondoutput port of the first 2×2 switch 380, and is used to monitorswitchover to the protection line.

[0047] Under normal operating conditions, a second 2×2 switch 370receives the protection signal received on its first input port andprovides that signal to its first output port. A second input port ofthe second 2×2 switch 370 receives the service signal, and provides theservice signal to a second output port of the second 2×2 switch 370. Asecond photodetector 360, for example a photodiode, is provided at thesecond output port of the second 2×2 switch 370, and is used to monitorloss of signal in the service path.

[0048] Under normal operating conditions, the first output port of thefirst 2×2 switch 380 is provided to a first input port of a third 2×2switch 340, and the first output port of the second 2×2 switch 370 isprovided to a second input port of the third 2×2 switch 340. The servicesignal received at the first input port of the third 2×2 switch 340 isprovided to a first output port of the third 2×2 switch 340, whichcorresponds to the main optical path 390. The second output port of thethird 2×2 switch 340, which corresponds to the protection optical path395, under normal operating conditions may be utilized to provideprotection data.

[0049] As explained above, the second photodiode 360 monitors theservice line signal under normal operating conditions, since the serviceline signal is provided to the second output port of the second 2×2switch 370 under those conditions. When the second photodiode 370detects an output level below a predetermined level, thereby indicatinga loss of signal in the service line, the controller 310 provides acontrol signal to the first 2×2 switch 380 so that the protection signal(received at the second input port of the first 2×2 switch 370) is nowprovided to the first output port of the first 2×2 switch 380. If theswitchover of the first 2×2 switch 380 occurs properly, this results inthe protection signal being provided to the first input port of thethird 2×2 switch 340, and thereby to the main optical path (coupled tothe first output port of the third 2×2 switch 340).

[0050] The first photodiode 320 monitors the switchover to theprotection line. After the first 2×2 switch 380 has been instructed tobe switched over, the first photodiode 320 detects whether the secondoutput port of the first 2×2 switch 380 transitions state. If there is amalfunction in the first 2×2 switch 380, the switchover instruction, asprovided to the first 2×2 switch 380 by the controller 310, may not haveresulted in proper switchover occurring at the first 2×2 switch 380. Inthat case, the third 2×2 switch 340 would be instructed by thecontroller 310 to couple its second input port to its first output port,and to couple its first input port to its second output port. This wouldresult in the protection signal, which is provided to the second inputport of the third 2×2 switch 340 by way of the second 2×2 switch 370,being provided to the main optical path that is coupled to the firstoutput port of the third 2×2 switch 340. The configuration shown in FIG.3 also allows for switches 370 and 340 to send service data through analternate route if first switch 380 fails.

[0051] Optional backup photodetectors 330, 350 are also shown in FIG. 3,and are provided in case the primary photodetectors 320, 360 aremalfunctioning. Similarly, an optional controller (not shown) may alsobe provided at the branch unit 300. With the configuration as shown inFIG. 3, a 6-7 dB loss in any one path from the input to the output ofthe branch unit 300 can be expected due to, for example, the splittersemployed therein.

[0052] The aforementioned advantages of the first embodiment are alsoapplicable to this second embodiment. Further, this second embodimentmay be implemented in various points throughout an optical network toprovide line switching in the event of a fiber cut. For example, branchunit 300 may be implemented in Baltimore, Md. between a node inWashington, D.C. and New York, N.Y. In the event of a fiber cut betweenBaltimore and Washington, branch unit 300 may switch optical fibers forjust that section, while not affecting the section from Baltimore, Md.to New York, N.Y. This second embodiment further provides additionalline monitoring and may be implemented with different switch technologythan employed in the first embodiment.

[0053] A third embodiment of a fiber optic system is shown by the blockdiagram of FIG. 4. The branch unit 400 comprises two 2×2 switches 420and 410. Service receive optical fiber 260 is optically connected to afirst input of a first 2×2 switch 410. Protect receive optical fiber 270is optically connected to a second input of a first 2×2 switch 410. Afirst output of the first 2×2 switch 410 is optically connected to afirst input of a second 2×2 switch 420. A second output of the first 2×2switch 410 is optically connected to a second input of a second 2×2switch 420.

[0054] In the third embodiment, under normal operating conditions,identical service and protection signals are received via optical fibers260 and 270, albeit on different input ports, of the first 2×2 switch410. Thus, the first 2×2 switch 410 receives, on its first input port,the primary or service information signals sent on the first branch path260. The first 2×2 switch 410 also receives, on its second input port,the backup or protection information signals sent on the second branchpath 270. In the preferred implementation of the third embodiment, thefirst and second 2×2 switches 410, 420 are preferably latching switches,which maintain their most recent switch position even if loss of poweroccurs.

[0055] At least two 2×2 switches 410 and 420 are provided in the branchunit 400 of the third embodiment to handle a case in which one of the2×2 switches 410, 420 is malfunctioning. In that regard, if the first2×2 switch 410 is malfunctioning in a manner such that the input fromthe first input port cannot be switched to the second output port of thefirst 2×2 switch 410, then the second 2×2 switch 420 is used to providethe proper signal onto primary transmission path 250, which correspondsto the output of the second 2×2 switch 420.

[0056] For example, assume that the service or primary signals areprovided on the first branch path and that the protection or backupsignals are provided on the second branch path. Under normal operatingconditions, the first 2×2 switch 410 and the second 2×2 switch 420 areoperated so that they are in a straight-through-output, andnot-crossed-output, state. That is, the first input port is coupled tothe first output port, and the second input port is coupled to thesecond output port, in the normal, straight-through-output state. Asshown in FIG. 4, this means that the service signals received at thesecond input port of the first 2×2 switch 410 are sent through thesecond output port of the first 2×2 switch 410, and then to the firstinput port of the second 2×2 switch 420, then to the output port of thesecond 2×2 switch 420, with the output port coupled to the main opticalpath 250.

[0057] Now, assume that a problem occurs on the first branch path inthat a fiber cut exists somewhere on the first branch path. In thatcase, no service signals are provided to the second input port of thefirst 2×2 switch 410 due to the fiber cut on the first branch path, andthus no signals are received at the second output port of the second 2×2switch 420. The photodetector 440 provides a “no signal” indication tothe processor 430, which then reconfigures the first and second 2×2switches 410, 420 to provide the protection signals on the second branchpath to the output port of the second 2×2 switch 420.

[0058] This reconfiguration can be done by one of two ways. The firstway is to set the first 2×2 switch 410 to a cross-connect mode, wherebythe first output port of the first 2×2 switch 10 is coupled to thesecond input port of the first 2×2 switch 410, and the second outputport of the first 2×2 switch 410 is coupled to the first input port ofthe first 2×2 switch 410. The second 2×2 switch 420 is left in thepass-through, non-cross-connected state. By this reconfiguration of thefirst 2×2 switch 410, the protection signals received from the secondbranch path are provided to the first branch path, which corresponds tothe output port of the second 2×2 switch 420.

[0059] Now, assume that even after this reconfiguration thephotodetector 440 still does not detect any signal being received at theoutput port of the second 2×2 switch 420. In this case, the first 2×2switch 410 may not have switched over to its cross-coupling mode eventhough it was instructed to do so by the processor 430. In this case,the second 2×2 switch 420 provides the cross-coupling needed to providethe protection signals to the output port of the second 2×2 switch 420.In particular, when the processor 430 is notified by the photodetector440 that a signal is still not being received at the output port of thesecond 2×2 switch 420, even after the processor 440 had instructed thefirst 2×2 switch 410 to change to a cross-coupling mode, then theprocessor 430 determines that the first 2×2 switch 410 ismalfunctioning, and thereby instructs the second 2×2 switch 420 tooperate in the cross-coupling mode. This effectively provides theprotection signals to the output port of the second 2×2 switch 420, theoutput port being coupled to the main optical path 250. Therefore, thefirst embodiment of the invention provides for non-interrupted servicewhen fiber cuts exist on the first branch path, but also when a 2×2switch in a branch unit is malfunctioning. An advantage of thisconfiguration is that losses due to splitters in the branch unit can beavoided.

[0060] A fourth embodiment of a fiber optic system is shown by the blockdiagram of FIG. 5. In this fourth embodiment, network protectionequipment (NPE) 540 is provided in optical communication with customerinterface equipment 530. NPE 540 comprises an array of branch units asdescribed by any one of the aforementioned embodiments in FIG. 24.

[0061] For example, NPE 540 may comprise an array of eight branch unitseach comprising three switches as described in a first embodiment.

[0062] On a transmit and receive side, there is an array of branch unitsin NPE 540, one for each of the WDM signals to be provided to fiber bays510 and 520. Each of the array of branch units of NPE 540 has aphotodiode detector at the output of the array, to thereby provideinformation to a processor so as to either switch one or more switchesin each array, if there is no signal detected at the output of thearray.

[0063] Referring now to FIG. 4, which shows a configuration that may beutilized for one WDM signal of NPE 540 according to the fourthembodiment, if the service line for that WDM signal is non-operative,then the output of the 2×2 switch 420 would indicate no signal present,as detected by photodiode 440. This information is provided to processor430, which provides control signals to switches 420, 410 to provide theprotection line for that WDM signal to the output of switch 420.

[0064] Similarly, a structure as shown in FIG. 2 or in FIG. 3 may beutilized for each of the WDM signals of NPE 540 according to the fourthembodiment.

[0065] As an alternative configuration of the fourth embodiment, onephotodiode may be utilized for more than one WDM signal, whereby outputsfrom a plurality of switches are provided to one photodiode, whereby alight tap from each of those switches is provided to the one photodiode.With this configuration, the photodiode can detect a problem in a groupof WDM signals, which may indicate a cut at a group level.

[0066] A fifth embodiment of a fiber optic system is shown by the blockdiagram of FIG. 6. NPEs 680 and 690 are provided in opticalcommunication with customer interface equipment (CIE) 695 and 685respectively. In this fifth embodiment, fiber bays 645 and 675communicate via service optical fiber 610 and fiber bays 655 and 665communicate via protect optical fiber 630. Typically fiber bays 645,675, 655, and 665 transmit and receive WDM signals via optical fibers610 and 630. Fiber bays 645, 675, 655, and 665 demultiplex the WDMsignals to single channel signals which are transmitted and received toNPEs 680 and 690 via optical fibers 660, 670, 640, and 650.

[0067] A fiber optic system according to this fifth embodiment issimilar in function to that described by the fourth embodiment. The maindifference between the two is that the NPEs 680 and 690 of the fifthembodiment operate on single channel signals, whereas NPE 540 of thefourth embodiment operates on WDM signals. Otherwise, the aforementioneddescription of NPE 540 also applies to NPEs 680 and 690 according tothis fifth embodiment.

[0068] A sixth embodiment of a fiber optic system is shown by the blockdiagram of FIG. 7. FIG. 7 depicts an array of switches 710, 720 and 730as may be implemented in an NPE as described in a fourth or fifthembodiment of FIGS. 5 and 6. In this configuration, taps 705, 715, and725 provide service optical fiber to a first input of switches 710, 720and 730 as shown. Taps 705, 715, and 725 further provide service opticalfiber to combiner 740 which outputs a combined service optical fiber tophotodetector 750. The functionality of the switches is similar to thatof a second embodiment as shown in FIG. 3, thus only the differenceswill be further described.

[0069] Photodetector 750 may provide information to a processor (notshown) regarding the status of the service optical fiber for a group ofswitches 710, 720, and 730. When the photodetector detects a drop inoptical strength due to signal loss, the processor may control switches710, 720, and 730 to provide connection via the protect optical fiber.

[0070] In a sixth embodiment, taps 705, 715, and 725 have different tapstrengths to allow photodetector 750 to tell which service optical fiberhas failed. For example, tap 705 may be a 1% tap, tap 715 may be a 5%tap, and tap 725 may be a 10% tap for a combined tap of 16%. When thephotodetector 750 detects a 10% loss, the service optical fiberconnected to switch 730 has failed and the processor can switch switch730 to provide connection via the protect optical fiber. Similarly, whenthe photodetector 750 detects a 6% loss, the service optical fibersconnected to switches 710 and 720 have failed and the processor canswitch switches 710 and 720 to provide connection via the protectoptical fibers. Other configurations, tap percentages, and the like maybe employed as would be readily apparent to one skilled in the art.Further, backup photodetector 760 may be provided in case of a failurein photodetector 750.

[0071] A seventh embodiment of a fiber optic system is shown by theblock diagram of FIG. 8. FIG. 8 depicts an array of switches 810, 820,830, 840, 850, 860, 870, 880, and 890 as may be implemented in an NPE asdescribed in a fourth or fifth embodiment of FIGS. 5 and 6. In thisexample, branch units comprise three switches 810, 840, and 850 in afirst branch unit, 820, 860, and 870 in a second branch unit, and 830,880, and 890 in a third branch unit similar to the branch unit describedin the second embodiment of FIG. 3. Couplers 835, 845, and 855 may beidentical to the couplers 290 and 295 in the second embodiment of FIG.3.

[0072] In this seventh embodiment, the second output of switches 850,870, and 890 are provided to optical combiner 865 which, in turn,provides a combined optical signal to photodector 809. Similarly, thesecond output of switches 840, 860, and 880 are provided to opticalcombiner 875 which, in turn, provides a combined optical signal tophotodetector 885.

[0073] Similar to photodetector 750 in a sixth embodiment of FIG. 7,photodetector 809 detects a failure on service optical fibers connectedto switches 850, 870, and 890. When a failure occurs in a serviceoptical fiber, a processor in communication with the photodetector 809can switch from service optical fiber to protect optical fiber.Similarly, photodetector 885 detects a failure on protect optical fibersconnected to switches 840, 860, and 880. When a failure occurs in aprotect optical fiber, a processor in communication with thephotodetector 885 can notify a user that the protect optical fiber hasfailed.

[0074] Optionally attenuators 805, 815, and 825 may be provided suchthat the amount of optical light received by combiner 865 from each ofthe service optical fibers is different. As aforementioned in a sixthembodiment of FIG. 7, using different % attenuators (FIG. 6 similarlyused varying % taps), photodetector 809 may be able to detect which ofthe service optical fibers has failed.

[0075] Optionally, attenuators may also be provided on the protectoptical fibers. Further, backup photodetectors 807 and 895 may beprovided in case of a failure in photodetectors 809 or 885.

[0076] A fiber optical architecture has been described according toseveral embodiments of the present invention. Many modifications andvariations may be made to the techniques and structures described andillustrated herein without departing from the spirit and scope of theinvention. Accordingly, it should be understood that the methods andapparatus described herein are illustrative only and are not limitingupon the scope of the invention. For example, the description ofcomponents and units as given above may be utilized for eitherland-based units or for underwater units. However, as will beappreciated by those skilled in the art, underwater units (e.g.,repeaters, switches and branch units) are typically hermetically sealed.

What is claimed is:
 1. A fiber optic system, comprising: a primarytransmission path provided from a source; a secondary transmission pathprovided from the source; a network protection unit coupled to theprimary and secondary transmission paths provided from the source, thenetwork protection unit comprising: a first 1×2 switch having a firstinput optically coupled to said primary transmission path, a secondinput optically coupled to said secondary transmission path, and anoutput; a second 1×2 switch having a second input optically coupled tosaid primary transmission path, a first input optically coupled to saidsecondary transmission path, and an output; and a third 1×2 switchhaving a first input optically coupled to said output of said firstswitch, a second input optically coupled to said output of said secondswitch, and an output optically coupled to an output transmission path;wherein, in a first mode of operation, the first and third switches areset to provide the primary signal to the output transmission path,wherein, in a second mode of operation, the first and third switches areset to provide the secondary signal to the output transmission path,wherein, in a third mode of operation, the second and third switches areset to provide the primary signal to the output transmission path, andwherein, in a fourth mode of operation, the second and third switchesare set to provide the secondary signal to the output transmission path.2. The fiber optic system of claim 1, wherein the fiber optic system isa submersible fiber optic system.
 3. The fiber optic system of claim 1,further comprising a photodiode optically connected to a tap of anoutput of the third 1×2 switch for detecting signal loss.
 4. The fiberoptic system of claim 3, further comprising a controller incommunication with the photodiode for controlling the first 1×2 switch,second 1×2 switch, and third 1×2 switch, wherein when the photodiodedetects a signal loss, the controller switches at least one of the three1×2 switches.
 5. The fiber optic system of claim 4, wherein the fiberoptic system automatically switches when a loss of signal in atransmission path occurs.
 6. The fiber optic system of claim 1, furthercomprising an array of switches in a network protection equipment. 7.The fiber optic system of claim 6, further comprising at least onephotodiode for monitoring the output of multiple switches.
 8. The fiberoptic system of claim 7, further comprising at least one attenuator perswitch, wherein each one of the attenuators has a different attenuation,and wherein the photodiode switches a failed switch based on the lossdetected attributable to that switch's attenuator.
 9. A fiber opticsystem, comprising: a primary transmission path provided from a source;a backup transmission path provided from the source; a branch unitprovided at a meeting point of the primary and backup transmissionpaths, the branch unit comprising: a first 2×2 switch having a firstinput optically coupled to said primary transmission path, a secondinput optically coupled to said secondary transmission path, a firstoutput, and a second output optically connected to a detector; a second2×2 switch having a second input optically coupled to said primarytransmission path, a first input optically coupled to said secondarytransmission path, a first output, and a second output optically coupledto a detector; a third 2×2 switch having a first input optically coupledto said first output of said first 2×2 switch, a second input opticallycoupled to said first output of said second 2×2 switch; a first output,and a second output; and a processor for receiving information from thedetectors regarding the detected signal strength at the second outputport of the first 2×2 switch and the second 2×2 switch, wherein thefirst 2×2 switch operates in either a first mode that provides inputreceived on its first input to its first output and input received onits second input to its second output, or a second mode that providesinput received on its first input to its second output and inputreceived on its second input to its first output, wherein the second 2×2switch operates in either a first mode that provides input received onits first input to its first output and input received on its secondinput to its second output, or a second mode that provides inputreceived on its first input to its second output and input received onits second input to its first output, and wherein the processor commandsthe first and second 2×2 switches to operate in one of the first mode ofoperation and the second mode of operation, based on the informationreceived from the detectors.
 10. The fiber optic system according toclaim 9, wherein the fiber optic system is a submersible fiber opticsystem.
 11. The fiber optic system of claim 9, wherein the fiber opticsystem automatically switches when a loss of signal in a transmissionpath occurs.
 12. The fiber optic system of claim 9, further comprisingan array of switches in a network protection equipment.
 13. The fiberoptic system of claim 12, further comprising at least one photodiode formonitoring the output of multiple switches.
 14. The fiber optic systemof claim 13, further comprising at least one attenuator per switch,wherein each one of the attenuators has a different attenuation, andwherein the photodiode switches a failed switch based on the lossdetected attributable to that switch's attenuator.
 15. A fiber opticsystem, comprising: a primary transmission path provided from a source;a backup transmission path provided from the source; a branch unitprovided at a meeting point of the primary and backup transmissionpaths, the branch unit comprising: a first 2×2 switch having a firstinput optically coupled to said primary transmission path, a secondinput optically coupled to said secondary transmission path, a firstoutput, and a second output; a second 2×2 switch having a first inputoptically coupled to said first input of said first 2×2 switch, a secondinput optically coupled to said second input of said second 2×2 switch,and an output optically coupled to a main transmission path; a detectoroptically coupled to an output of said second 2×2 switch; and aprocessor in communication with said detector for controlling said first2×2 switch and said second 2×2 switch, wherein the first 2×2 switchoperates in either a first mode that provides input received on itsfirst input to its first output and input received on its second inputto its second output, or a second mode that provides input received onits first input to its second output and input received on its secondinput to its first output, wherein the second 2×2 switch operates ineither a first mode that provides input received on its first input toan output, or a second mode that provides input received on its secondinput to an output, and wherein the processor commands the first 2×2switch and second 2×2 switch to operate in one of the first mode ofoperation and the second mode of operation, based on the informationreceived from the detector.
 16. The fiber optic system according toclaim 15, wherein the fiber optic system is a submersible fiber opticsystem.
 17. The fiber optic system of claim 15, wherein the fiber opticsystem automatically switches when a loss of signal in a transmissionpath occurs.
 18. The fiber optic system of claim 15, further comprisingan array of switches in a network protection equipment.
 19. The fiberoptic system of claim 18, further comprising at least one photodiode formonitoring the output of multiple switches.
 20. The fiber optic systemof claim 19, further comprising at least one attenuator per switch,wherein each one of the attenuators has a different attenuation, andwherein the photodiode switches a failed switch based on the lossdetected attributable to that switch's attenuator.
 21. A method ofproviding fiber optic signals on a fiber optical network, the methodcomprising: providing, from a source, a primary signal on a primarytransmission path; providing, from the source, a backup signal on abackup transmission path; receiving the primary and backup signals onthe primary and backup transmission paths, respectively, and outputtingonly one of the primary and backup signals onto an output port thatcorresponds to a main optical path, by way of at least two switches;detecting a signal characteristic on the main optical path; anddetermining, based on said signal characteristic, whether to operate ina first mode of operation, in which the primary signal is provided tothe main optical path, or in second mode of operation, in which thebackup signal is provided to the main optical path.
 22. The method ofclaim 21, wherein the fiber optical network is a submersible fiberoptical network.
 23. The method of claim 21, wherein said signalcharacteristic is signal quality.
 24. The method of claim 21, whereinsaid signal characteristic is signal strength.
 25. An opticalcommunication system comprising: a source for providing a primaryoptical signal on a primary transmission path and a backup opticalsignal on a backup transmission path; a branch unit for receiving saidprimary optical signal and said backup optical signal and forselectively switching one of said primary optical signal and said backupoptical signal onto a main optical path, wherein said selection of saidprimary optical signal and said backup optical signal is made based upona measurement taken along are of said primary, backup and main opticalpaths.
 26. The optical communication system of claim 25, wherein saidbranch unit includes at least three switches and a controller.